1. Field of the Invention
The invention relates to a method and a device for measuring the positions of centres of curvature of optical surfaces of a single- or multi-lens optical system.
2. Description of the Prior Art
In the manufacture of high-quality multi-lens optical systems, the lenses must be aligned relative to one another with high accuracy. In order to be able to carry out such alignment, it is necessary to determine the positions of the optical surfaces by measurement. Even if the position accuracy is not checked during alignment of the lenses, such measurements are routinely carried out at least within the scope of quality control.
An important geometrical parameter in the measurement of multi-lens optical systems is the positions of the centres of curvature of the optical surfaces. Ideally, the centres of curvature lie exactly on a common reference axis, which should generally coincide with the axes of symmetry of the lens mounts holding the lenses. In real optical systems, however, the centres of curvature are distributed randomly about that reference axis as a result of manufacturing and mounting tolerances. If the distances of the centres of curvature from the reference axis are too great, the imaging properties of the optical system deteriorate intolerably.
From DE 10 2004 029 735 A1 there is known a method for measuring centres of curvature of optical surfaces of a multi-lens optical system, wherein the positions of the centres of curvature of the individual optical surfaces are measured in succession by means of an autocollimator. For each optical surface, the measurement is preferably carried out several times at different azimuthal angular positions of the optical system. The first surface for which the position of the centre of curvature is measured is the surface that is closest to the autocollimator. As soon as the position of the centre of curvature of this first surface has been determined, the subsequent second surface is measured. However, the first surface influences the measurement of the second surface. The optical effect of the first surface must therefore be taken into consideration mathematically when determining the position of the centre of curvature of the second surface. When taking into consideration the optical effect of the first surface, recourse is made to the design data of the first surface, in particular to the desired radius of curvature and the desired distance from the second surface (that is to say the centre thickness of the first lens). The previously measured position of the centre of curvature of the first surface is additionally taken into consideration mathematically.
The same procedure is followed for all further surfaces. Accordingly, the measured positions of the centres of curvature of all preceding optical surfaces and, in addition, the design data are taken into consideration in the mathematical evaluation.
In this known method, the centre of curvature of an optical surface to be measured must always be situated in the image plane of the autocollimator, whereby the refractive effect of any optical surfaces of the optical system situated upstream in the beam path is to be taken into consideration. It is thus ensured that the measuring light strikes the optical surface to be measured perpendicularly and is reflected back on itself. Only then is the measuring object of the autocollimator, which can be, for example, a cross-wire, imaged sharply on a spatially resolving light sensor of the autocollimator. After each measurement of the position of a centre of curvature, the focal length of the autocollimator must therefore be readjusted. In general, this is carried out by displacing one or more lenses along the optical axis of the autocollimator.
In particular in the case of industrial measuring tasks, where a very large number of optical systems of the same type are to be measured in a short time, a considerable portion of the total measuring time is taken up by this repeated adjustment of the focal length.
Similar problems also arise when measuring the two centres of curvature of a single lens, for example in order to determine its optical axis therefrom. Here too, the focal length must be altered once for each measuring operation.